How Do I Create An C# Asp.Net Button For Save And Validate?

public string this[string columnName]
{
    get
    {
        Result = null;
        if (columnName == "FirstName")
        {
            if (String.IsNullOrEmpty(FirstName))
            {
                Result = "Please enter first name";
            }
        }
        else if (columnName == "LastName")
        {
            if (String.IsNullOrEmpty(LastName))
            {
                    Result = "Please enter last name";
            }
        }
        else if (columnName == "Address")
        {
            if (String.IsNullOrEmpty(Address))
            {
                Result = "Please enter Address";
            }
        }
        else if (columnName == "City")
        {
            if (String.IsNullOrEmpty(City))
            {
                Result = "Please enter city";
            }
        }
        else if (columnName == "State")
        {
            if (State == "Select")
            {
                Result = "Please select state";
            }
        }
        else if (columnName == "Zip")
        {
            if (String.IsNullOrEmpty(Zip))
            {
                Result = "Please enter zip";
            }
            else if (Zip.Length < 6)
            {
                Result = "Zip's length has to be at least 6 digits!";
            }
            else
            {
                bool zipNumber = Regex.IsMatch(Zip, @"^[0-9]*$");
                if (zipNumber == false)
                {
                    Result = "Please enter only digits in zip";
                }
            }
        }
        else if (columnName == "IsValid")
        {
            Result = true.ToString();
        }

        return Result;
    }
}